1. Chapter 6: User-Defined Functions I
C++ Programming: Program Design Including Data Structures, Fifth Edition
Chapter 6: User-Defined Functions I

2. Objectives In this chapter, you will:
Learn about standard (predefined) functions and discover how to use them in a program
Learn about user-defined functions
Examine value-returning functions, including actual and formal parameters
Explore how to construct and use a value-returning, user-defined function in a program

3. Introduction Functions are like building blocks
They allow complicated programs to be divided into manageable pieces
Some advantages of functions:
A programmer can focus on just that part of the program and construct it, debug it, and perfect it
Different people can work on different functions simultaneously
Can be re-used (even in different programs)
Enhance program readability

4. Introduction (cont'd.) Functions Called modules
Like miniature programs
Can be put together to form a larger program

5. Predefined Functions
In algebra, a function is defined as a rule or correspondence between values, called the function’s arguments, and the unique value of the function associated with the arguments
If f(x) = 2x + 5, then f(1) = 7, f(2) = 9, and f(3) = 11
1, 2, and 3 are arguments
7, 9, and 11 are the corresponding values

6. Predefined Functions (cont'd.)
Some of the predefined mathematical functions are:
sqrt(x)
pow(x, y)
floor(x)
Predefined functions are organized into separate libraries
I/O functions are in iostream header
Math functions are in cmath header

7. Predefined Functions (cont'd.)
pow(x,y) calculates xy
pow(2, 3) = 8.0
Returns a value of type double
x and y are the parameters (or arguments)
The function has two parameters
sqrt(x) calculates the nonnegative square root of x, for x >= 0.0
sqrt(2.25) is 1.5
Type double

8. Predefined Functions (cont'd.)
The floor function floor(x) calculates largest whole number not greater than x
floor(48.79) is 48.0
Type double
Has only one parameter

9. Predefined Functions (cont'd.)

10. Predefined Functions (cont'd.)

11. Predefined Functions (cont'd.)

12. Predefined Functions (cont'd.)
Example 6-1 sample run:

13. User-Defined Functions
Value-returning functions: have a return type
Return a value of a specific data type using the return statement
Void functions: do not have a return type
Do not use a return statement to return a value

14. Value-Returning Functions
To use these functions you must:
Include the appropriate header file in your program using the include statement
Know the following items:
Name of the function
Number of parameters, if any
Data type of each parameter
Data type of the value returned: called the type of the function

15. Value-Returning Functions (cont'd.)
Because the value returned by a value-returning function is unique, must:
Save the value for further calculation
Use the value in some calculation
Print the value
A value-returning function is used in an assignment or in an output statement
One more thing is associated with functions:
The code required to accomplish the task

16. Value-Returning Functions (cont'd.)

17. Value-Returning Functions (cont'd.)
Heading: first four properties above
Example: int abs(int number)
Formal Parameter: variable declared in the heading
Example: number
Actual Parameter: variable or expression listed in a call to a function
Example: x = pow(u, v)

18. Syntax: Value-Returning Function
functionType is also called the data type or return type

19. Syntax: Formal Parameter List

20. Function Call

21. Syntax: Actual Parameter List
The syntax of the actual parameter list is:
Formal parameter list can be empty:
A call to a value-returning function with an empty formal parameter list is:

22. return Statement
Once a value-returning function computes the value, the function returns this value via the return statement
It passes this value outside the function via the return statement

23. Syntax: return Statement
The return statement has the following syntax:
In C++, return is a reserved word
When a return statement executes
Function immediately terminates
Control goes back to the caller
When a return statement executes in the function main, the program terminates

24. Syntax: return Statement (cont’d.)

25. Function Prototype
Function prototype: function heading without the body of the function
Syntax:
It is not necessary to specify the variable name in the parameter list
The data type of each parameter must be specified

26. Function Prototype (cont'd.)

27. Function Prototype (cont'd.)

28. Value-Returning Functions: Some Peculiarity

29. Value-Returning Functions: Some Peculiarity (cont'd.)

30. Value-Returning Functions: Some Peculiarity (cont'd.)

31. Example 6-5: Palindrome Number
A nonnegative integer is a palindrome if it reads forward and backward in the same way
Examples: 5, 44,

32. Example 6-5: Palindrome Number (cont’d.)

33. Flow of Execution
Execution always begins at the first statement in the function main
Other functions are executed only when they are called
Function prototypes appear before any function definition
The compiler translates these first
The compiler can then correctly translate a function call

34. Flow of Execution (cont'd.)
A function call results in transfer of control to the first statement in the body of the called function
After the last statement of a function is executed, control is passed back to the point immediately following the function call
A value-returning function returns a value
After executing the function the returned value replaces the function call statement

35. Programming Example: Largest Number
The function larger is used to determine the largest number from a set of numbers
Program determines the largest number from a set of 10 numbers
Input: a set of 10 numbers
Output: the largest of 10 numbers

36. Programming Example: Program Analysis
Suppose that the input data is:
Read the first number of the data set
Because this is the only number read to this point, you may assume that it is the largest number so far and call it max
Read the second number and call it num
Compare max and num, and store the larger number into max

37. Programming Example: Program Analysis (cont'd.)
Now max contains the larger of the first two numbers
Read the third number and compare it with max and store the larger number into max
max contains the largest of the first three numbers
Read the next number, compare it with max, and store the larger into max
Repeat this process for each remaining number in the data set

38. Programming Example: Algorithm Design
Read the first number
Because this is the only number that you have read, it is the largest number so far
Save it in a variable called max
For each remaining number in the list
Read the next number
Store it in a variable called num
Compare num and max

39. Programming Example: Algorithm Design (cont'd.)
For each remaining number in the list (cont'd.)
If max < num
num is the new largest number
update the value of max by copying num into max
If max >= num, discard num; that is, do nothing
Because max now contains the largest number, print it

40. Summary Functions (modules) are miniature programs
Divide a program into manageable tasks
C++ provides the standard functions
Two types of user-defined functions: value-returning functions and void functions
Variables defined in a function heading are called formal parameters
Expressions, variables, or constant values in a function call are called actual parameters

41. Summary (cont'd.)
In a function call, the number of actual parameters and their types must match with the formal parameters in the order given
To call a function, use its name together with the actual parameter list
Function heading and the body of the function are called the definition of the function
A value-returning function returns its value via the return statement

42. Summary (cont'd.)
A prototype is the function heading without the body of the function; prototypes end with the semicolon
Prototypes are placed before every function definition, including main
User-defined functions execute only when they are called
In a call statement, specify only the actual parameters, not their data types